LAB VIIi: OHMS
LAW
Problem: What is the relationship between current, and
resistance when there is a constant voltage in an electric circuit.
Materials: Voltmeter, Ammeter,
5 resistors,light bulb, wire, power source.
Procedure:
1) Copy Table
I. And Table II into you
lab notebook.
2) Assemble the
circuit as shown in the diagram. Leave the end clips to insert
the various resistors.
3) Measure each
resistor with the ohmeter. Do not change the dial. Keep the dial
set to ohms (W) . Put each resistor on a piece of paper and label each
resistor.
4) Insert the
1000 ohm resistor in the circuit.
5) Have your
teacher check your circuit. Turn on the switch to power the 1
volt battery. Read the current on the ammeter. Record the current
in table I (see results section). Unclip the 1000 ohm resistor
and insert 1500 ohms and repeat. Continue increasing the resistance.
For values you do not have you will have to combine two or
three resistors in series.
If time permits....
6) Measure the
resistance of a light bulb turned off.
7) Put the light
bulb into the circuit in place of a resistor. When the light bulb
is turned on record the voltage and current in the circuit. Use
ohms law to calculate the resistance of the light bulb when turned
on.
Results:
Table I.
V= 1.5 volts
Currents (amps) Resistance (Amps)
________ 1000
________ 1500
________ 2000
________ 3000
________ 5000
Table II.
V= 3.0 volts
Currents (amps)
Resistance (Amps)
________ 1000
________ 1500
________ 2000
________ 3000
________ 5000
Graphs: Plot
your data from Table I and Table II on the same axis. Label the
xaxis resistance (ohms)
and the yaxis current (ma milliamperes)
Discussion:
1) What are the
independent and dependent variables?
2) How are the
variables changing with relationship to each other?
3) What happens
to the dependent variable when the independent variable increases?
decreases?
4) How does the
relationship shown in this experiment compare with other relationships
you have so far seen?
5) How does the
equation for this relationship compare with those of other equations
you have studied?
6) What is the
equation for ohms law? How does this relate to your graph.
Applications:
1) Explain how
a light dimmer works.
2) How does a
fuse work?
3) If instead
of keeping the voltage constant you kept the resistance constant,
then plotted voltage vs current, what shape graph would you get?
What would the slope of the graph be?
4) If you measured
the resistance of the light bulb explain the difference when the
bulb was off and on. How does this affect the efficiency in an
electric circuit?
